In U.S. Pat. Nos. 4,278,514 and 4,406,755 are disclosed electroplating solutions for the deposition of palladium. Those solutions contain palladium in the form of a soluble organo-palladium complex formed from an inorganic palladium salt and an organic polyamine complexing agent. In the operation of those electroplating solutions, the organic polyamine remains free in the solution after the palladium has been electrodeposited. In the free state, the organic polyamine tends to increase stress in the electrodeposits. Accordingly, it is common operating practice to remove the polyamine as it is formed by circulating the solution through a filtering apparatus containing activated carbon. This has the disadvantage that activated carbon also removes a small amount of the usable organopalladium complex from the solution, and thus represents an additional cost.
A further characteristic of the electroplating solutions of U.S. Pat. Nos. 4,278,514 and 4,406,755 is that they are not easily adaptable to the electrodeposition of palladium alloy deposits. This is because the chemical stabilities of organometallic complexes formed from polyamines vary widely with the nature of the metal incorporated. In electrodepositing alloys, the discharge potentials of the metals to be co-deposited should be as close to each other in value as practicable. This is difficult or impossible if the chemical stabilities of the soluble metallic species present vary markedly from each other.
It is often desirable to produce alloys of palladium with other metals by electrodeposition. Wrought alloys of palladium with silver, for example, and both wrought and electrodeposited alloys of palladium with nickel have proven to be useful in electrical and electronic applications. Recently, B. Sturzenegger and J. Cl. Puippe: Platinum Metal Rev., 28:117 (1984) reported the electrodeposition of alloys of palladium with silver using ammoniacal solutions and U. Cohen, K. R. Walton and R. Sard: J. Electrochem. Soc., 131:2489 (1984) similarly described the use of acidic solutions containing large quantities of chloride ion to achieve the same purpose. U.S. Pat. Nos. 4,465,563 and 4,478,692 describe the electrodeposition of alloys of palladium with silver from solutions containing an excess of a strong organic or inorganic acid. In commercial practice, alloys of palladium with nickel are almost invariably electrodeposited from alkaline ammoniacal solutions, concerning which there exists a voluminous patent literature, both United States and foreign.
All of the solutions herein referred to for palladium alloy electrodeposition are, by virtue of excess acidity, alkalinity, or the presence of large concentrations of ammonium or chloride ions, chemically aggressive toward most base metals, i.e., nickel, copper, or copper alloys, onto which the desired palladium alloy electrodeposits are ordinarily applied. Consequently, the electroplating processes require that a strike, usually of gold, silver or palladium, be applied to the work in order to protect it from attack by the electroplating solution. Such a strike requirement represents both an additional cost and a loss of process freedom, as the presence of a strike coating dissimilar to both the base metal and the electrodeposit raises the possibilities of galvanic interaction in the event of electrodeposit porosity, or of interdiffusion at elevated temperatures. It is clearly desirable, then, that in any electroplating process, the electroplating solution should be as chemically nonaggressive as possible toward the work to be electroplated, so that the requirement for a strike deposit can be minimized or eliminated entirely.
In view of the foregoing, it is an object of this invention to provide an electroplating solution for the deposition of palladium, said solution being free of organic polyamine complexing agents.
It is a further object that the electroplating solution thus constituted should, upon addition of a suitable soluble species of an alloying metal, be capable of depositing an alloy of palladium with the said alloying metal.
It is a further object that the solution or solutions of this invention be capable of operating in a range of pH which is neither very strongly acid nor alkaline.
It is yet a further object that electrodeposits produced in accordance with this invention should be bright, and, to as great a degree as possible, free of such defects as porosity, cracking, and excessive stress.